dB-linear voltage-to-current converter

ABSTRACT

A dB-linear voltage-to-current (V/I) converter that is amenable to implementation in CMOS technology. In a representative embodiment, the dB-linear V/I converter has a voltage scaler, a current multiplier, and an exponential current converter serially connected to one another. The voltage scaler supplies an input current to the current multiplier based on an input voltage. The current multiplier multiplies the input current and a current proportional to absolute temperature and supplies the resulting current to the exponential current converter. The exponential current converter has a differential MOSFET pair operating in a sub-threshold mode and generating an output current that is proportional to a temperature-independent, exponential function of the input voltage.

This application is a continuation of U.S. application Ser. No. 12/363,849, entitled “dB-LINEAR VOLTAGE-TO-CURRENT CONVERTER”, filed Feb. 2, 2009, the entirety of which is hereby incorporated herein by reference to be considered part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electronics and, more specifically, to voltage-to-current (V/I) converters having an exponential transfer function.

2. Description of the Related Art

This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.

An exponential (or dB-linear) voltage-to-current (V/I) converter is a key component for the design of automatic gain-control (AGC) circuits, which are used in a variety of applications, such as wireless communications devices, hearing aids, and disk drives. A representative AGC circuit employs an exponential V/I converter in the feedback loop that controls the gain of a variable-gain amplifier (VGA). The basic form of an AGC loop, as shown in FIG. 6, consists of an alternating current (a. c.) coupled input 1 followed by a variable gain amplifier (VGA) 3 which drives a low pass filter 5 followed by an a. c. coupled output 7 to subsequent circuitry, with a feedback loop from the output 7 of the low pass filter through a peak detector 9 to an exponential voltage-to-current converter 11. Converter 11 provides as output a control signal that controls the gain of the VGA 3. The exponential characteristic of the V/I converter enables the AGC circuit to advantageously have a substantially constant settling time for a variety of initial input-signal conditions, which is very desirable for the above-specified applications. Additional details on the use of exponential V/I converters in AGC circuits can be found, e.g., in U.S. Pat. No. 6,369,618, which is incorporated herein by reference in its entirety.

One problem with exponential V/I converters is that they are not straightforwardly amenable to implementation in CMOS technology. More specifically, unlike bipolar transistors, which have an inherent exponential transfer characteristic, MOSFET transistors have a square-law transfer characteristic in strong inversion. As a result, designing a CMOS V/I converter that exhibits an exponential transfer characteristic and has other desirable properties is relatively difficult.

SUMMARY OF THE INVENTION

Problems in the prior art are addressed by various embodiments of an exponential (or dB-linear) voltage-to-current (V/I) converter that is amenable to implementation in CMOS technology. In a representative embodiment, the exponential V/I converter has a voltage scaler, a current multiplier, and an exponential current converter serially connected to one another. The voltage scaler supplies an input current to the current multiplier based on an input voltage. The current multiplier multiplies the input current and a current proportional to absolute temperature and supplies the resulting current to the exponential current converter. The exponential current converter has a differential MOSFET pair operating in a sub-threshold mode and generating an output current that is proportional to a temperature-independent, exponential function of the input voltage. Advantageously, the exponential V/I converter can be implemented to have a dB-linear operation range as wide as about 40 dB.

According to one embodiment, provided is a device having (A) a current multiplier that multiplies a first current and a current proportional to absolute temperature to generate a second current; and (B) an exponential converter that applies an exponential transfer function to the second current to generate an output current. The exponential transfer function depends on a thermal voltage. Temperature dependence of the current proportional to absolute temperature counteracts temperature dependence of the thermal voltage to cause the output current to be proportional to a temperature-independent, exponential function of the first current over an operating range of the device.

According to another embodiment, provided is a method having the steps of: (A) multiplying a first current and a current proportional to absolute temperature to generate a second current; and (B) applying an exponential transfer function to the second current to generate an output current. The exponential transfer function depends on a thermal voltage. Temperature dependence of the current proportional to absolute temperature counteracts temperature dependence of the thermal voltage to cause the output current to be proportional to a temperature-independent, exponential function of the first current over a specified operating range.

According to yet another embodiment, provided is a device having (A) means for multiplying a first current and a current proportional to absolute temperature to generate a second current; and (B) means for applying an exponential transfer function to the second current to generate an output current. The exponential transfer function depends on a thermal voltage. Temperature dependence of the current proportional to absolute temperature counteracts temperature dependence of the thermal voltage to cause the output current to be proportional to a temperature-independent, exponential function of the first current over an operating range of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and benefits of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which:

FIG. 1 shows a block diagram of an exponential (or dB-linear) voltage-to-current (V/I) converter according to one embodiment of the invention;

FIG. 2 shows a circuit diagram of a voltage scaler that can be used in the exponential V/I converter of FIG. 1 according to one embodiment of the invention;

FIG. 3 shows a circuit diagram of a current multiplier that can be used in the exponential V/I converter of FIG. 1 according to one embodiment of the invention;

FIG. 4 shows a circuit diagram of an exponential current converter that can be used in the exponential V/I converter of FIG. 1 according to one embodiment of the invention; and

FIG. 5 shows a circuit diagram of a current limiter that can be used in the exponential V/I converter of FIG. 1 according to one embodiment of the invention.

FIG. 6 is a block diagram of a typical AGC circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a block diagram of an exponential (or dB-linear) voltage-to-current (V/I) converter 100 according to one embodiment of the invention. Converter 100 receives input voltage V_(in) and converts it into output current I_(out) so that there is an exponential relationship between the input voltage and the output current. As further described below, converter 100 is amenable to implementation in CMOS technology and is advantageously capable of maintaining the exponential transfer characteristic over a relatively wide (e.g., about 40 dB) output-current range.

Converter 100 has a voltage scaler 110 that conditions input voltage V_(in) for further processing in the subsequent circuit blocks of the converter. More specifically, voltage scaler 110 scales input voltage V_(in) and adds bias voltage V_(bias) to the scaled voltage according to Eq. (1): V ₁₁₀ =V _(bias) +γV _(in)  (1) where V₁₁₀ is the output voltage of the voltage scaler, and γ is a scaling factor. In one embodiment, one or both of bias voltage V_(bias) and scaling factor γ are programmable so that output voltage V₁₁₀ remains in an optimal range for the entire variability range of input voltage V_(in). In a representative embodiment, |V_(bias)|0.2V and γ≈0.5.

Converter 100 applies output voltage V₁₁₀ to resistive load R₀, which drives current I₁ through that load. In effect, the combination of voltage scaler 110 and resistive load R₀ serves as a voltage-to-current converter that converts input voltage V_(in) into current I₁. The subsequent signal processing in converter 100 is current-based and converts current I₁ into output current I_(out).

Converter 100 further has a current multiplier 120 whose output current I₂ is expressed according to Eq. (2): I ² =ηI ₁ I _(PTAT)  (2) where η is a constant, and I_(PTAT) is a current proportional to absolute temperature (PTAT). In effect, current multiplier 120 generates output current I₂ by multiplying the input current (i.e., current I₁) and current I_(PTAT) As further described below, the temperature proportionality of current I_(PTAT) is utilized to make the exponential transfer characteristic of converter 100 substantially temperature independent and enable the converter to operate accurately and reliably in a variety of ambient conditions without a thermostat.

Output current I₂ produced by current multiplier 120 is applied to an exponential current converter 130 that converts output current I₂ into output current I₃ according to Eq. (3): I ₃ =Aexp(σI ₂ /V _(T))  (3) where A and σ are constants, and V_(T) is the thermal voltage (=k_(B)T/q, where k_(B) is the Boltzmann constant, T is temperature in Kelvin, and q is the electron charge). Eqs. (2) and (3), taken together, indicate that current multiplier 120 and exponential current converter 130 work together to provide a substantially temperature-independent, exponential transfer function between currents I₁ and I₃. More specifically, according to Eqs. (2) and (3), the argument of the exponent in Eq. (3) contains current I_(PTAT) and thermal voltage V_(T) in the nominator and denominator, respectively. Since both current I_(PTAT) and thermal voltage V_(T) are linear functions of temperature, their temperature dependencies cancel each other, thereby causing the exponential transfer function between currents I₁ and I₃ to be substantially temperature independent.

Exponential current converter 130 applies output current I₃ to a current limiter 140, where it is processed to generate output current I_(out). More specifically, current limiter 140 imposes lower limit I_(min) and upper limit I_(max) onto current I₃. If the magnitude of current I₃ is smaller than lower limit I_(min), then current limiter 140 forces I_(out)≧I_(min). If the magnitude of current I₃ is larger than upper limit I_(max), then current limiter 140 forces I_(out)≈I_(max). If the magnitude of current I₃ is between lower limit I_(min) and upper limit I_(max), then current limiter 140 forces I_(out)≈I₃.

FIG. 2 shows a circuit diagram of a voltage scaler 200 that can be used as voltage scaler 110 according to one embodiment of the invention. Voltage scaler 200 has a reference current source 202, an operational amplifier 204, and four resistors R₁-R₄ interconnected as shown in FIG. 2. If R₁>>R₂, then voltage scaler 200 implements the transfer function defined by Eq. (1), wherein:

$\begin{matrix} {V_{bias} \approx {I_{ref}\frac{R_{2}\left( {R_{3} + R_{4}} \right)}{R_{3}}}} & \left( {4a} \right) \\ {\gamma \approx \frac{R_{2}\left( {R_{3} + R_{4}} \right)}{R_{1}R_{3}}} & \left( {4b} \right) \end{matrix}$ where I_(ref) is the current generated by reference current source 202. Note that resistor R₁ is a programmably variable resistor, which enables programmability of scaling factor γ. In one embodiment, reference current source 202 is a programmably variable current source, which enables programmability of bias voltage V_(bias).

FIG. 3 shows a circuit diagram of a current multiplier 300 that can be used as current multiplier 120 according to one embodiment of the invention. Current multiplier 300 has two nested differential amplifiers, each having an active current-mirror load. The active devices of the first (outer) differential amplifier are MOSFET transistors M5 and M6, and the load of that differential amplifier is the current mirror formed by transistors MOSFET M1 and M2. Similarly, the active devices of the second (inner) differential amplifier are transistors MOSFET M7 and M8, and the load of that differential amplifier is the current mirror formed by MOSFET transistors M3 and M4. The gates of transistors M5 and M7 are both electrically connected to a common node having floating voltage V_(x). The gates of transistors M6 and M8 are both electrically connected to a common node that receives reference voltage V_(ref).

In one embodiment, reference voltage V_(ref) is supplied by a programmable reference-voltage source (not explicitly shown in FIG. 3). The voltage source is programmed to set a value of V_(ref) so that there is a desired relationship between output current I₂ and input voltage V_(in). In particular, V_(ref) is selected from a voltage range, wherein, if V_(ref) changes, then the transfer function between output current I₂ and input voltage V_(in) is translated along the voltage axis without changing its slope.

Current multiplier 300 further has reference current sources 302 and 304 that function as tail supplies of the outer and inner differential amplifiers, respectively. Current source 302 is designed to generate reference current I_(bgap) that does not depend on the technological process used in the fabrication of current multiplier 300 or on the temperature of the current multiplier. In one embodiment, current source 302 can be implemented, as known in the art, using a conventional bandgap circuit. Current source 304 is designed to generate temperature-dependent PTAT current I_(PTAT) (see also Eq. (2)). In one embodiment, current source 304 can be a PTAT current source disclosed, e.g., in U.S. Patent Application Publication No. 2008/0284493, which is incorporated herein by reference in its entirety. In one configuration, current source 304 generates current I_(PTAT) so that, at room temperature (T₀=298 K), I_(PTAT)=I_(bgap).

Operation of transistors M5-M8 in current multiplier 300 is described by Eqs. (5a)-(5d), respectively:

$\begin{matrix} {\frac{I_{bgap} + I_{1}}{2} = {\frac{1}{2}\mu_{n}C_{ox}\frac{W_{1}}{l_{1}}\left( {V_{x} - V_{a} - V_{th}} \right)^{2}}} & \left( {5a} \right) \\ {\frac{I_{PTAT} + I_{2}}{2} = {\frac{1}{2}\mu_{n}C_{ox}\frac{W_{2}}{l_{2}}\left( {V_{x} - V_{b} - V_{th}} \right)^{2}}} & \left( {5b} \right) \\ {\frac{I_{bgap} - I_{1}}{2} = {\frac{1}{2}\mu_{n}C_{ox}\frac{W_{1}}{l_{1}}\left( {V_{ref} - V_{a} - V_{th}} \right)^{2}}} & \left( {5c} \right) \\ {\frac{I_{PTAT} - I_{2}}{2} = {\frac{1}{2}\mu_{n}C_{ox}\frac{W_{2}}{l_{2}}\left( {V_{ref} - V_{b} - V_{th}} \right)^{2}}} & \left( {5d} \right) \end{matrix}$ where μ_(n) is the mobility of electrons; C_(ox) is the capacitance of the oxide layer; W₁ and I₁ are the width and length, respectively, of transistors M5 and M6; W₂ and I₂ are the width and length, respectively, of transistors M7 and M8; V_(a) and V_(b) are the voltages indicated in FIG. 3; and V_(th) is the threshold voltage. If transistors M5-M8 are implemented so that

${\frac{\left( {W_{1}/l_{1}} \right)}{\left( {W_{2}/l_{2}} \right)} = 1},$ then V_(x)≈V_(ref) and Eqs. (5a)-(5d) reduce to Eq. (2), wherein η=(I_(bgap))⁻¹.

If current multiplier 300 is used in V/I converter 100 to implement current multiplier 120, then the following relationship exists between input voltage V_(in) and current I₁: I ₁ =αV _(in) +i _(c)  (6) where α=γ/R₀ and i_(c)=(V_(bias)−V_(ref))/R₀. Note that, for a given configuration of V/I converter 100, α and i_(c) are constants.

FIG. 4 shows a circuit diagram of an exponential current converter 400 that can be used as exponential current converter 130 according to one embodiment of the invention. Exponential current converter 400 has a differential pair of MOSFET transistors M1 and M2 that are configured to operate in a sub-threshold mode (also referred to as a cut-off or weak-inversion mode). The gates of transistors M1 and M2 are electrically connected through resistor R₁₂. The gate of transistor M2 receives bias voltage V_(bias1) from a bias-voltage generator 410. Transistor M3 serves as a tail supply for the differential pair. A current source 402 drives reference current I_(ref1) through transistor M2. A current source 404 and transistor M4 are used to appropriately bias transistors M2 and M3.

As known in the art, drain-to-source current I_(ds) in a MOSFET transistor operating in a sub-threshold mode varies exponentially with gate-to-source voltage V_(gs), as expressed by Eq. (7):

$\begin{matrix} {I_{ds} \approx {I_{0}{\exp\left( \frac{V_{gs} - V_{th}}{{nV}_{T}} \right)}}} & (7) \end{matrix}$ where I₀ is a constant; and n=1+C_(D)/C_(ox), where C_(D) is the capacitance of the depletion layer. Applying Eq. (7) to transistor M2, one finds that:

$\begin{matrix} {I_{{ref}\; 1} \approx {I_{0}{\exp\left( \frac{V_{{bias}\; 1} - V_{s} - V_{th}}{{nV}_{T}} \right)}}} & (8) \end{matrix}$ where V_(s) is the voltage indicated in FIG. 4. Further applying Eq. (7) to transistor M1 and then using Eq. (8), one finds that:

$\begin{matrix} {{I_{3} \approx {I_{0}{\exp\left( \frac{V_{{bias}\; 1} + {I_{2}R_{12}} - V_{s} - V_{th}}{{nV}_{T}} \right)}}} = {I_{{ref}\; 1}{\exp\left( \frac{I_{2}R_{12}}{{nV}_{T}} \right)}}} & (9) \end{matrix}$ Note that Eq. (9) is equivalent to Eq. (3), wherein A=I_(ref1) and σ=R₁₂/n

If current multiplier 300 and exponential current converter 400 are used in V/I converter 100 to implement current multiplier 120 and exponential converter 130, respectively, then the following relationship exists between input voltage V_(in) and current I₃:

$\begin{matrix} {{I_{3} = {B\;{\exp\left( {\beta\; V_{in}} \right)}}}{where}{B = {{I_{{ref}\; 1}{\exp\left( \frac{R_{12}I_{PTAT}i_{c}}{{nV}_{T}I_{bgap}} \right)}\mspace{14mu}{and}\mspace{14mu}\beta} = {\frac{\alpha\; R_{12}I_{PTAT}}{{nV}_{T}I_{bgap}}.}}}} & (10) \end{matrix}$ Note that, for a given configuration of V/I converter 100, B and β are constants that do not depend on the temperature because the temperature dependencies of current I_(PTAT) and thermal voltage V_(T) substantially cancel each other. Thus, Eq. (10) indicates that V/I converter 100 employing current multiplier 300 and exponential current converter 400 provides a temperature-independent, exponential transfer function between input voltage V_(in) and current I₃. In addition, current multiplier 300 and exponential current converter 400 are advantageously capable of exhibiting a dB-linear transfer function over a relatively wide (e.g., about 40 dB) operation range because the exponential current converter invokes the inherent exponential characteristic of MOSFET transistors in the sub-threshold operating mode.

FIG. 5 shows a circuit diagram of a current limiter 500 that can be used as current limiter 140 according to one embodiment of the invention. Current limiter 500 has reference current sources 502 and 504, an operational amplifier 506 with a feedback network, and two current mirrors formed by transistors M1, M4, M5, and M6. Reference current I_(ref3) generated by current source 502 sets the minimum output current for current limiter 500. Reference current I_(ref2) generated by current source 504 sets the maximum output current for current limiter 500.

Current source 502 sets the minimum output current for current limiter 500 because it is directly connected to an output terminal 508 of the current limiter. As a result, output current I_(out) has at least a current component corresponding to reference current I_(ref3). Hence, output current I_(out) does not drop below the value of I_(ref3) even if current I₃ becomes zero.

Current source 504 sets the maximum output current for current limiter 500 in the following manner. Transistors M1 and M2 have substantially the same size, which causes the value of I_(ref2) to set the ON/OFF level for transistor M3. More specifically, if current I₃ is smaller than I_(ref2), then operational amplifier 506 holds transistor M3 in the OFF state. As a result, the two current mirrors formed by transistors M1, M4, M5, and M6 force output current I_(out) to mirror current I₃. However, if current I₃ is greater than I_(ref2), then operational amplifier 506 turns ON transistor M3, which sinks the excess current and causes the current flowing through transistor M1 to remain at the value of I_(ref2). The two current mirrors then mirror the current flowing through transistor M1 onto output terminal 508, thereby substantially forcing output current I_(out) not to exceed the value of I_(ref3).

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Although certain embodiments of the invention have been described in reference to NMOS technology, the invention is not so limited. Various circuits of the inventions can also be implemented using the PMOS technology, the bipolar CMOS technology, and various non-MOS technologies, including implementations in an integrated circuit. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims.

As used herein, the term dB-linear means that, when plotted on a logarithmic scale over an operation range, the output current is a substantially linear function of the input voltage (or current), wherein slope of the linear function does not deviate from a specified value by more than about ±5%.

Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range.

It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.

Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.”

Also for purposes of this description, the terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements.

Transistors are typically shown as single devices for illustrative purposes. However, it is understood by those with skill in the art that transistors will have various sizes (e.g., gate width and length) and characteristics (e.g., threshold voltage, gain, etc.) and may consist of multiple transistors coupled in parallel to get desired electrical characteristics from the combination. Further, the illustrated transistors may be composite transistors.

As used in the claims, the terms “source,” “drain,” and “gate” should be understood to refer either to the source, drain, and gate of a MOSFET or to the emitter, collector, and base of a bi-polar device when the present invention is implemented using bi-polar transistor technology. 

What is claimed is:
 1. A method comprising: multiplying a first current and a current proportional to absolute temperature to generate a second current; applying an exponential transfer function to the second current to generate an output current; and imposing at least one of an upper limit and a lower limit on the output current, imposing the lower limit including directly coupling a first reference-current source to an output terminal and generating a first reference current with the first reference-current source to set the lower limit, and imposing the upper limit including coupling a second reference-current source to a first input of an operational amplifier, connecting an output and a second input of the operational amplifier via a feedback loop, applying the output current to the second input of the operational amplifier, coupling first and second current mirrors between the second input of the operational amplifier and the output terminal, and generating a second reference current with the second reference-current source to set the upper limit.
 2. The method of claim 1 wherein the exponential transfer function depends on a thermal voltage.
 3. The method of claim 2 wherein temperature dependence of the current proportional to absolute temperature counteracts temperature dependence of the thermal voltage to cause the output current to be proportional to a temperature-independent and exponential function of the first current over an operating range.
 4. The method of claim 3 wherein on a scale of the output current, the operating range is at least about 40 dB.
 5. The method of claim 1 further including generating the first current based on an input voltage, and over an operating range, the output current is proportional to a temperature-independent and exponential function of the input voltage.
 6. The method of claim 5 wherein the input voltage includes a programmable scaling factor and a programmable bias voltage.
 7. The method of claim 1 wherein the step of multiplying includes providing first and second differential amplifiers, each having a corresponding current mirror as a load, coupling a first current source as a tail supply of the first differential amplifier, and coupling a second current source as a tail supply of the second differential amplifier, the second current source generating the current proportional to absolute temperature, the first differential amplifier receiving the first current, and the second differential amplifier outputting the second current.
 8. The method claim 7 wherein a reference current from the first current source is independent of technological process used for fabrication of a current multiplier and of temperature.
 9. The method of claim 8 wherein at room temperature, the reference current from the first current source is approximately equal to the current proportional to absolute temperature.
 10. The method of claim 1 wherein the step of applying includes providing a differential transistor pair that includes a first transistor and a second transistor, providing a resistor that electrically connects a gate of the first transistor and a gate of the second transistor, and driving a reference current through the second transistor, the second current flowing through the resistor, the output current flowing through the first transistor.
 11. The method of claim 10 wherein each of the first and second transistors is a MOSFET transistor.
 12. The method of claim 11 wherein the step of applying further includes configuring the MOSFET transistors to operate in a sub-threshold mode to enable the exponential transfer function.
 13. An automatic gain control circuit comprising: a variable gain amplifier configured to receive a time varying input signal, to receive a control signal, and to output an output signal, an amplitude of the output signal varying in response to the control signal; a low pass filter configured to receive the output signal and to drive a circuit output terminal; a peak detector circuit electrically coupled to the circuit output terminal and configured to output a voltage indicating when the output signal is outside a predetermined range; and an exponential voltage-to-current converter circuit configured to receive an output of the peak detector circuit as an input and to output the control signal to drive control circuitry for controlling a gain of the variable gain amplifier, the exponential voltage-to-current converter circuit including a current multiplier that multiplies a first current and a current proportional to absolute temperature to generate a second current, an exponential converter that applies an exponential transfer function to the second current to generate an output current, and a current limiter that imposes at least one of an upper limit and a lower limit on the output current, the current limiter including a first reference-current source directly coupled to a converter output terminal, an operational amplifier, a second reference-current source coupled to a first input of the operational amplifier, an output and a second input of the operational amplifier connected via a feedback loop, the output current applied to the second input of the operational amplifier, and first and second current mirrors coupled between the second input of the operational amplifier and the converter output terminal, the converter output terminal outputting a limited current such that a second reference current generated by the second reference-current source sets the upper limit.
 14. The automatic gain control circuit of claim 13 wherein the exponential transfer function of the exponential voltage-to-current converter circuit depends on a thermal voltage, and temperature dependence of the current proportional to absolute temperature counteracts temperature dependence of the thermal voltage.
 15. The automatic gain control circuit of claim 13 wherein the exponential transfer function of the exponential voltage-to-current converter circuit provides a linear characteristic output to control the variable gain amplifier such that the automatic gain control circuit has approximately constant settling time independent of temperature.
 16. The automatic gain control circuit of claim 13 wherein the exponential voltage-to-current converter circuit further includes a differential transistor pair including a first transistor and a second transistor, a resistor that electrically connects a gate of the first transistor and a gate of the second transistor, and a current source that drives a reference current through the second transistor, the second current flowing through the resistor, and the output current flowing through the first transistor.
 17. The automatic gain control circuit of claim 16 wherein each of the first and second transistors is a MOSFET transistor that operates in a sub-threshold mode to enable the exponential voltage-to-current converter circuit to apply said exponential transfer function.
 18. The automatic gain control circuit of claim 13 wherein the exponential voltage-to-current converter circuit is in a feedback loop that controls the gain of the variable gain amplifier, the control signal is based on the output current, and the output current is proportional to a temperature-independent and exponential function of the first current over an operating range.
 19. A device comprising: means for multiplying a first current and a current proportional to absolute temperature to generate a second current; means for applying an exponential transfer function to the second current to generate an output current, the exponential transfer function depending on a thermal voltage, and temperature dependence of the current proportional to absolute temperature counteracting temperature dependence of the thermal voltage to cause the output current to be proportional to a temperature-independent and exponential function of the first current over an operating range; and means for imposing at least one of an upper limit and a lower limit on the output current, the means for imposing including a first reference-current source directly coupled to an output terminal such that a first reference current generated by the first reference-current source sets the lower limit, the means for imposing further including an operational amplifier, a second reference-current source coupled to a first input of the operational amplifier, an output and a second input of the operational amplifier connected via a feedback loop, the output current applied to the second input of the operational amplifier, and first and second current mirrors coupled between the second input of the operational amplifier and the output terminal, the output terminal outputting a limited current such that a second reference current generated by the second reference-current source sets the upper limit.
 20. The device of claim 19 wherein on a scale of the output current, the operating range is at least about 40 dB.
 21. The device of claim 19 further including means for generating the first current based on an input voltage, and over the operating range, the output current is proportional to a temperature-independent and exponential function of the input voltage.
 22. The device of claim 21 wherein the input voltage includes a programmable scaling factor and a programmable bias voltage.
 23. The device of claim 19 wherein the means for multiplying includes first and second differential amplifiers, each having a corresponding current mirror as a load, a first current source coupled as a tail supply of the first differential amplifier, and a second current source coupled as a tail supply of the second differential amplifier, the second current source generating the current proportional to absolute temperature, the first differential amplifier receiving the first current, and the second differential amplifier outputting the second current.
 24. The device of claim 23 wherein a reference current from the first current source is independent of technological process used for fabrication of a current multiplier and of temperature.
 25. The device of claim 24 wherein at room temperature, the reference current from the first current source is approximately equal to the current proportional to absolute temperature. 